Wash the potatoes well to remove any dirt and debris. Then slice the potatoes thinly and place them in at least a 6-quart soup pot along with the celery root.

Peel and roughly chop the onions, and peel the garlic. Place them in the pot along with the potatoes, celery root and cashews. Pour over chicken stock, and sprinkle with the sage, salt and pepper. Bring to a boil, and then reduce the flame to a simmer.

Cook the soup for 1 hour. Puree the soup, and then return to the pot. Simmer the soup for an additional 15 minutes, and correct the seasoning.